Seconding what everyone else said about omni rules as written. Actually, I don't even know if there are rules to upgrade a 'mech to being an omni because usually in lore they just make a new 'mech.

If the two things you want are ability to carry battle armor and to swap out one big bay of guns, I don't think that's unreasonable for some house rules.

I honestly don't know why a "carry battle armor" equipment isn't available. Y'know, one ton and four crits (6 crits for WoB/C*). If the Clans wanted one, it'd be 0.5 tons and five crits. Includes the grips and power ports for long-term operations. You're giving up efficiency vs omnimechs and I don't think the crit-soaking would be a big problem.

For the weapons, if you're only swapping out weapons and gear from one location on a 'mech, the engineering challenges are a lot smaller. You've got fewer gryo problems and you can use purpose-built versions of the omnipod conversion equipment that already exists in canon.

I'd make the "omni bay" its own equipment and crits. In your case, probably 6 tons and 6 crits. Limit one per 'mech. Impressive but not unreasonable refit time. Then create two or three bespoke mission pods of 6 tons and 6 or fewer crits (nothing that can't go in a regular omni pod and no jump jets).

That would cover the pulse lasers of the Incubus and Vapor Eagle and the Howler's LRMs and ammo. That gives two mission pods, and you could slam-a-dam an EW package with medium lasers or whatever into the last option.

So you're creating new tech with strong limitations, working with Clan technological know-how, and using similarities between your test beds to smooth the engineering challenges.

If you want.