Is it possible to make a Legacy Weapon for a Deathwatch character? I know about the Signature Wargear rules, but I was wondering if I could modify the Legacy Weapon creation chart to make a deathwatch weapon as a step above the Signature Wargear stuff?

Probably. I suspect a more or less straight port, utilizing Renown instead of Infamy would be fairly straightforward.

I'd also presume that weapon you want to make a Legacy Weapon would either need to be a Signature Wargear item or otherwise part of your standard issue for class/etc.