Assuming they're not using custom styles, converting them to NeoLemmix is very easy (as long as you're not using custom graphic sets - though even if you are, many of the most common ones have NeoLemmix versions); there's even a mass convert option in the NeoLemmix Editor. You'll need to first copy your Lemmini styles from Lemmini's "styles" folder, to NeoLemmix Editor's "styles/Lemmini" folder. Then, you can use the Mass Convert option under the Tools menu.
Actually building them into an NXP pack is a bit trickier but still fairly simple - check out the
Flexi Toolkit (there's a tutorial on how to use it on that page, too). Alternatively, just distribute the LVL files themself.
Do be aware that (as far as I know) the level database does not support NeoLemmix levels. You'll have to upload them somewhere else - perhaps as an attachment on the forums. (I really need to get around to making a proper NeoLemmix Level Database website...)
I'm not at home at the moment so can't check the full details for the mass conversion (been a while since I've used or worked on it), but if no one else helps you first, I'll be glad to help you when I am at home.

Don't forget - always test the levels after conversion; NeoLemmix's mechanics are not exactly the same as Lemmini's; if anything they're closer to those of DOS / Lemmix (but still fairly different even from that).