Enhancer Bails Work. Generally, an enhancer is a bail which opens to allow the necklace, pearls, for instance, to be inserted and the enhancer is closed to secure it to the necklace. The types of closures are an important consideration. Some only "snap" shut. Others only have a small "figure 8" shaped safety that folds from one side over a small bead on the other to make the closure.
The "figure 8" safety is one commonly seen on bangle bracelets, gold necklaces with box or cylindrical clasps and on bracelets. The most secure type of enhancer both "snaps" shut and has the "figure 8" safety for added assurance the enhancer will not come open while being worn on the necklace.
Generally, a 14k gold enhancer bail will run from about $50 to $90 in the most common sizes. Fancier ones and added stones will certainly increase the price. If the pendant you have will need a loop soldered on to fit the enhancer, or if the enhancer must be soldered to the pendant, the cost will rise a little. A simple loop soldered on will be about $14 to $18 in my neck of the woods. Solder directly to the enhancer will run from $10 to perhaps $15, including soldering and refinishing. This is added to the cost of the enhancer.
Prices certainly may vary from location to location. Keep in mind, in the usa most jewelers can get the findings, parts, like enhancer bails, for essentially the same prices, regardless of location. For any pendant, and enhancer of the right size is needed to maintain balance in the look of the jewelry. The enancer should fit the necklace and not overpower or look too small for the pendant.
Then again, a necklace of pearls should also balance in size with the pendant. I have seen some enhancers which are much too large for the pendant part of the jewelry and detract from the beauty. If the necklace requires a larger opening, you might choose a longer and more "slim" enhancer which can accommodate a larger necklace but is narrow enough to look in balance with the pendant.
The best thing I can suggest is to go to a jewelry shop and ask to see enhancers. Generally, not many are kept in stock since it is very difficult to predict what a particular customer may need. You can generally see life sized images in catalogs from suppliers. At this time, the store should be able to tell you what labor or modifications to your pendant are needed to fit the enhancer.
There is alot of people that are looking for a some custom jewelery like a gold jerusalem cross pendant with small diamonds or cz at the center of each cross. This unusual form of cross is not likely to be found in stock in jewelry stores. Sometimes these stores will have cross jewelry but most will not be made in gold.
My suggsent is, you will likely have to go to jewelry stores and ask if they can special order the cross for you. At this time of the year, the sooner you can do this the better. Once you get the cross, any competent jeweler should be able to add a stone to the cross.
I did find one jewelry supplier who has a Jerusalem Cross in their religious jewelry catalog. This company is Stuller Findings. The company sells only to businesses but so many jewelry stores use Stuller for supplies and various jewelry items it should not be too difficult to find a store with a Stuller account. The cross could then be ordered. The order would need to specify white gold.
Alternative suggestion. There are many of these crosses listed on Internet sellers sites along with other jewelry that you might like. I do not know if white gold is available or not and that will take a search and perhaps an inquiry to the seller. If you can get the cross that way, still, a local jeweler should have little problem setting a stone in the cross. Whether stones may be set in the side crosses will depend on how the cross is made in the first place. Center should be no problem in the larger cross.
Custom Work: A jewelry store who advertises "custom work" should be able to make the cross if you cannot find one. This approach will be more expensive and it might be pushing to get this custom work done by the time that you need it at. If you try the custom direction, please download and print or otherwise get pictures of the cross for the jeweler to see. This will be needed to get the cross done properly and initially to work up and estimate of cost.
There are certainly some other suppliers that jewelry stores use who will have a Jerusalem cross available. When checking stores, I recommend local stores first and not mall chain stores. Local stores often have a lot more flexibility for special ordered items such as the cross. Local stores are often more eager to place a special order than some chain stores appear to be.