I'm thinking about starting my campaign as a Dark Heresy game at rank1, then kicking the party into the Expanse once they have spent 5,000xp. What is the feasibility of this? Would it work? Are there any problems that I should be aware of? Thoughts and ideas are very welcome and appreciated!

A 2/3rd completed Rank 5 Dark Heresy character is going to have a lot more Breadth than an equivalent Rank 1 Rogue Trader character. A Dark Heresy character has cheaper advances and a much wider base to select from by the time they'd ready to "start" in Rogue Trader. On the other hand Rogue Trader characters are probably going to have higher stats since they have a +5 advantage over DH characters, and their origin paths give them even more boosts into certain areas. As your games progress though an 8th Rank Dark Heresy character will wipe the floor with an 8th rank Rogue Trader character in a straight fight, which no 8th level Rogue Trader would ever engage in.

If you really want to integrate the games, I would suggest letting characters build their origins using Rogue Trader (seriously, it's a great origin system), or just play Dark Heresy up to that point. Once they transition into Rogue Trader, re-scale the XP (and what you award) appropriately to try and keep things balanced. You could go with a straight doubling from Dark Heresy to Rogue Trader, but I prefer to keep it approximately consistent with what RT presents for Skill/Talent costs.

100 XP Dark Heresy Cost => 200 XP Rogue Trader Cost

200 XP Dark Heresy Cost => 500 XP Rogue Trader Cost

300 XP Dark Heresy Cost => 700 XP Rogue Trader Cost

500 XP Dark Heresy Cost => 1000 XP Rogue Trader Cost

This also helps converting DH and RT alternate ranks back and forth between the systems.