NavCanada recently issued an Aeronautical Information Circular that all controllers should be aware of: AIC 12/21 https://www.navcanada.ca/en/5aic_eng_2021_12.pdf
To summarize, due to operational irregularities that have occurred, controllers cannot create a transition from an open STAR to an RNAV Approach.
(An open STAR is any STAR that does not share a common waypoint with an IFR approach procedure, but usually ends with either a vector or a hold.)
If there is no common waypoint between the approach and the STAR, the controller MUST VECTOR the aircraft towards the final approach course.
As an example, consider an aircraft on the DISCO arrival for CYYJ (Victoria, BC). (All diagrams are posted below.) If the aircraft is planning an ILS to 09, there is no transition point to the ILS 09 that is on the STAR, so the controller MUST vector the airplane to final. Conversely, if the aircraft is planning ILS to 27, LAFFO is on the ILS 27 and thus a controller may clear the aircraft "Cleared ILS Rwy 27, LAFFO transition."
Example of what is not permitted: "After OMINU, direct BRANY, cleared ILS 09." This transition must be made using vectors.
By contrast, many airports are now publishing transitions to the approach, such as CYWG (Winnipeg, MB). If an aircraft is on the BEFAN2 for the ILS 36, note that the waypoint BOTUR is on both procedures, the STAR and the approach. Therefore, a controller can clear an aircraft "Cleared ILS Z Rwy 36, BOTUR transition."
Basically, we cannot create transitions that have not been published.
(Another further aside, and one of the reasons we can't do this, is that turns made at a fix using RNAV cannot exceed 90degrees, whether published or controller made. We cannot issue a direct-to a fix that would require an aircraft to make a greater than 90degree turn to join the next leg of the flight plan or instrument procedure. Published procedure turns and Radius-To-Fix legs are exempt from this limit as they are specifically evaluated for their turn radius.)
I hope I made this explanation fairly clear. please feel free to ask questions if something does not make sense.