So in a mixed Pathfinder - Starfinder game the question comes up, how do you handle magic weapons?
Just treating them as normal Pathfinder weapons will not make them awesome. Seriously, I would toss a +3 longsword away around level 8 when the better weapons come online. Starfinder exists in this strange reality where melee weapons can do a LOT of damage, so here is what I did, and this works within Hero Lab Online.
For every "plus" in damage, add one base damage die. Make these custom melee weapons so you can change the name, and add the hit modifier after the fact, so this +1 longsword would actually attack at melee +2 and do 2d8. Ignore the +2 damage of Pathfinder and only up the base dice.
This makes weapons up to a +5 do 6d8 S damage with a hefty to-hit modifier, making them viable all the way up to level 18. The white-star plasma sword at level 18 does 8d8 E & F damage but does not get the very nice and possibly unbalancing +5 to-hit modifier. Less damage, way better hit, feels like a trade-off, especially when you consider the level 20 plasma sword doing 10d8 E & F damage.
That +5 sword is still gonna be viable against tougher targets even at level 20 since it will hit 25% more for a little more than half damage of the big one. Still, against a nearly invulnerable foe, that may make all the difference. Also, if the sword is flaming it will do S& F damage, so more modifications are possible given the weapon's enchantments.
This will make all those magic swords, daggers, axes, and other ancient weapons lying around those ruins and tombs in my current campaign all the more valuable and attractive to players, collectors, and anyone else wanting to get their hands on some old-school pointy steel.