You really have to play it by ear.
Since the focus of Travller is not usually the "kill monsters and take their stuff" model of D&D, assigning XP is much more a matter of the Referee using his (or her) discretion. D&D3 does provide guidelines for calculating challenge ratings for NPCs based on their level, so you could use that as a guide, but I think it would be less than ideal. The technology is a real leveler.
I'd suggest that XP should be awarded for achieving adventure goals and for good roelplaying, even if it runs counter to the adventure goals (A romantic PC who trusts the attractive female NPC even though his player knows she's a spy.)
The amount of XP awarded depends on the pace of advancement you want and the level of the PCs. If the average PC is around 7th level (need 7000 to reach 8th level) and you want the PCs to go up one level per six sessions or so, then you need to give about 1000 to 1500 XP per session, on average. You could give less and add a big bonus at the end of a set of seeiosn for resolving the plot, if you prefer.
Really old-school Traveller would not award XP at all; PCs didn't really advance after character generation.