You can use skins that were developed prior to mojoPortal 2.5 with mojoPortal 2.5. The folder structure of the skins we release now is different but that's just a preference. mojoPortal doesn't care about that folder structure. As long as your skins are inside of the /data/sites/<site number>/skins folder and the root of your skin folder has layout.master, styleconfig, and theme.skin, you're good to go.
You do need to check out the Important Skin Changes documentation though. There are some settings changes you should be aware of.
Hope this helps,