Actually there's several ways to atain bankai or other "transformations" the first two are already set in place because they are the easiest to control...however, I believe if a person is clear enough in their roleplays he or she could attain bankai or other transformation in midst of one of their encounters. The standard rules of 3+ pages involvement still applies (this means you cant rp in one page and wait till three more pages occur before you post and say you attained bankai.)