White wine will work. Depending on the amount added, it may lighten the elderberry a bit without diluting it.
If you're going to sweeten the elderberry (which I recommend -- bone dry elderberry is unpleasant), you can stabilize and add a commercial fruit juice. Pretty much any juice will do, although ones without added sugar are better. But since you can't go out, whatever you have has to be good enough.
Do you understand what stabilization is? [I'm proceeding as if you don't -- the information may benefit others.]
When a wine has completed fermentation, potassium sorbate and potassium metabisulfite can be added to prevent a renewed fermentation. This must be done when back sweetening a wine, else the remaining yeast will resume fermentation, eating the new sugar.
If you don't have sorbate on hand, all is not lost. Add juice and be prepared for the wine to ferment again (you will probably need to move the wine back into the primary fermenter). But add more than you need, e.g., if you need a cup of top up, I'd add at least a quart of juice. This way you will have top up for later rackings as well.
Regardless of what I said previously, if all else fails, use water. Water may dilute the wine, but air will destroy it.
