Training with specialist skills I imagine would end up being easier than general tactical skills, since these specialists are usually organized into dedicated cells. Training could be done by setting up what is essentially a training cell, which supplies training in the same way a bombmaking cell would supply IEDs. Commanders could pass down orders to a cell stating that their most knowledgeable member is to be transferred to Cell T (The training cell, though they should only be told it is an honorable and elite cell, both to maintain OPSEC and to make sure they don't send a bumblefuck)– if there are many cells of the same specialty then Commanders could request something like a resume be sent on behalf of each cell's most knowledgeable individual. Once the training cell is established with experts from most specialist fields then classes can begin: new and under-performing individuals can be reported by the cell leader to a regional commander, who can then send a request for training up to the Commander, who will notify Cell T. Once there are enough individuals for a class, all cells that have reported an individual in need of training will be told that those individuals will report to a certain building on X date and expect them to be gone for Y weeks. The trainer should give a fake name and give the impression that he is simply a lone benefactor helping the organization, perhaps claiming some former education or professional experience to justify all his knowledge.
Alternatively, intelligent members could be identified and sent to an over-performing cell as an "apprentice" in order to learn how they operate and build a training plan around those operations (working of course with the cell leader to ensure accuracy,) which could either be used to establish Cell T using similarly intelligent "apprentices" or the training plans could be sent to every cell to be used for intracell training. I should also mention that this all assumes that this is a relatively large organization. In smaller organizations training would probably be intracell.
This is all off-the-cuff, there are probably several glaring errors here, feel free to point them out.