stupid question, I know but…some of the alternate career ranks from that game might, under some circumstances, fit into a Rogue Trader game and I was wondering if I could just plug 'em right in or should I sit down and crunch numbers to rebalance 'em a bit before giving the go ahead. thoughts/opinions on the matter from any rules gurus out there would be appreciated.

The major problem with it is that compared to Dark Heresy, Rogue Trader seems to place more emphasis on acquisitions and awesome toys than individual skill ranks.

Let's take The Lathe Worlds, which is the Mechanicus book that was the one one of my players had been waiting for forever, but was unfortuantely introduced at the end of our Dark Heresy campaign. Its Rank 2 advance of Mech-Assassin is awesomely flavourful, but only offers 2700 EXP of advances compared to 4800 EXP for the same Explorator rank it would replace. Rank 2 in Rogue Trader has 3000XP total, so you can see it's a little underpowered.

My suggestion for alternate ranks from Dark Heresy is as follow. The earliest Rank of the advance corresponds to the Rank of Rogue Trader. Any skill or talent that costs 100 in DH now costs 200. Anything costing 200 costs 500. Anything costing 300 costs 750. Anything costing more than 300 costs 1000. Obviously you can modify this as you see fit, but this puts the purchase power of a Dark Heresy career more in-line with a Rogue Trader career.

Using this modification scheme, the Mech-Assassin now has 5650 EXP of advances, which makes it closer to what a Rogue Trader advance should be.

As Erathia points out, XP costs are generally twice as expensive in Rogue Trader as in Dark Heresy.

In general I am not a big fan of alternate career ranks, I don't mind players taking the background and lore from an alternating career as inspiration for their character, and I might even let them buy some of the key skills/talents as elite advances if they do a good job of convincing me that the character works in that role. I will not go along with a skill/talent-grab attempt.

As a general rule, no. The advancement schemes and experience costs - and even some of the skills and talents - are different in Rogue Trader compared to Dark Heresy.

I would advise against using any of the alternate careers from Dark Heresy "as is", but a lot of them make for very nice inspiration for homebrewed conversions.