I converted area movement to movement by "squares" (penciled on to black-core foamecore/painted granny grate) each of which were 1 1/2" across. Marines moved 6 squares; terminators moved 4 squares while jump equipped Marines moved 12" as they could jump across open space.
I used conventional 40K templates for flammers and explosions (I added simple grenade rules missing from Overkill)
Statistical attributes and abilities were essentially straight out of the Overkill rules (as best I could translate) with 10" for Combat range and 20" for Maximim range. Hormagaunt/Sisters of Battle/Mechanicus Canine stats were house-ruled.
I managed the Genestealers as the GM positioning them in ambush to best impede, challenge or obstruct the Deathwatch…in the spirit if the card system… which I suppose would have worked as well.
To introduce random positive or negative events players rolled one 6 sided "fate die" along with their to hit roll. I translated fate die (+) results into either a positive event (die reroll, added movement, added attack, enemy pushback, save modifier, wound recovery…) or a negative (-) event (weapon jam, pushback, slip and fall, save modifier, obstructed shot, additional enemy appearance…).
Other than that the simplicity and "mission" style objectives of the game were retained. If anyone has done stat conversions for other forces in the 40K universe not included in the game I would love to get my hands on them….