So first of all I'm very-very beginner in Blender. Previously I have used Source Filmmaker. In SFM you had an option to apply IK rig to models and after that detach it. Now in Blender I have a model (not own) which has standard IK rig in it. My goal is to pose the model with FK only and after that re-enable IK rigs to that newly created pose (similar like in SFM). I can't do that because if I'm re-enabling IK it always puts back the actual body-part to its default position. Am I missing something? Thank you in advance for your help!
You may be able to achieve something similar to this with the auto ik setting, but I'm not sure as I don't really use it.
IK/FK switching while preserving pose is difficult in blender as it currently stands.
The only way I've seen it done is by having 3 sets of bones, one for the FK pose, one for the IK and one to actually deform the mesh. A driver is then created to determine which chain controls the deform chain. Scripts are then provided to snap the FK chain to the IK chain's position and vice versa.
The above technique is used in the Rigify rig that comes as an addon with blender.
Also as an aside, why do you want to enable IK after posing? This will cause the rig to use IK interpolation(straight line between poses) between poses rather than FK? Most of the time you will actually want FK interpolation(arcs between poses), the only time you normally want IK if the end of the chain is explicitly fixed (e.g. a foot planted on the ground), and in these cases it is often easier to pose by IK anyway.
What i usually do when I need to keep the transform but lose the ik is one of two things.
Thing 1: use the 3D cursor to place dummy objects in place of where you want the bones to be, and then move them in to position manually one by one, or... (since this can get a bit cumbersome if you have a lot of IK/constraints going on)
Thing 2: Keyframe all the bones to whatever poses you need and export an FBX. Upon export It will convert everything to FK. All you need to do then is either bring it into a new project and go from there, or if you absolutely must have it applied to the object in the original project you exported it from, you can import the FBX back into your project and then transfer (copy/paste) its animation/action keyframes back over to the original model via the dope sheet.