By "absorb," I take it that you mean "replace all existing Templars and distribute their powers among loyal new recruits?" I can't see many existing Templars agreeing to this plan since their job is to police mages and will only go to war if the Chantry decrees it is their duty. Remember that Templars are chosen for religious zeal, and so it will be no small task to convince them to do anything the Chantry hasn't approved of. Fighting Tevinter at the behest of an idealist, secular warmonger is not what they signed up for, and I don't think you will be able to convince them to die on behalf of Tevinter slaves...
By absorb I mean tear the organization apart and intergrate its members into the Inquisition, spreading the them among different units. THose who don't wish to join, may leave. THose who may try and get back at the Mages or have a similar disposition to Alrik from DA2 will be executed or will have "accidents". Whatevers best for the situation. With the most zealous, I would form them into Mage/Templar teams of mage hunters who's purpose is to hunt dangerous mages like serial killers.
They may not care about the slaves, but I can easily just play up the whole "All Magisters are Blood Mages" angle. I could hire Fenris to explain it to them!
There's no doubt more to it and if it's an option in DA:I, I'm hoping Bioware covers what I missed 