I'm not sure there is a capital version of "varepsilon." I can't find any character in Unicode with "epsilon" in it's name that looks like varepsilon, except the lowercase symbols. Capital epsilons always look like a western "E".
Can you provide an image of the symbol, or a link to an article where it's used? Maybe it goes under a different name.
Anyway, if you can find the symbol you want, you can define your own name for it in OOo Math (see Tools > Catalog), or you can insert it using it's Unicode character code.
AOO/LO 4 • Linux • Fedora 21