Clicking the slot brings up a list of eligible pieces to fill the slot, but in the case of melee slots, some might be classified as weapons, in spite of saying Hand in the name. Took some experimenting to get an Assault Greatsword and Assault Arena Fist on my Banshee-2P. You might have to pick a Hand first, as mechs can't dual melee weapons.

you'll find this useful, and anyone who wants to find a list of the variants:

https://www.reddit.com/r/Mechwarrior5/comments/vknmpp/what_mechs_can_weild_melee_weapons/