I believe they can, but are a lot rarer. Honestly, I could be wrong here though.

Reclaim Assimilated Borg Drones does indeed give a DOff on a critical success.

Where does Reclaim assimilated borg drones show up?

I've only seen it twice on Holodeck, both times in Gamma Orionis.

Excellent guide. Qapla' OP

I got my first and only prisoner DOFF, Fed side, from a critical success on the Uncover Intelligence Leak mission.

Commendation tier rewards
Each faction has a vendor that sells unique DOffs: Lieutenant Ferra at Starfleet Academy and Lieutenant S'stas at Klingon Academy. However, these DOffs are locked until certain commendation ranks are achieved. Each tier of each commendation category has a different set of 5 or more DOffs available at this vendor. Reaching a tier awards a token for one free DOff in the set, and unlocks the others for purchase with dilithium. Green DOffs cost 1000 dilithium, blue DOffs cost 5000 dilithium, and purple DOffs cost 12000 dilithium.

I just got my medical and military commendations (2500 points) and have acquired my two DOFFS. I cannot however purchase more for dilithium as stated above (taken from the guide). Do I need to reach the next tier (15000 points) ?

Right to show I am not afraid of being called stupid (for this is what I am in this case) I will leave this here. When you get your commendation DOFF, the unlocked ones for Di are at the bottom of the list .... scroll down

Thanks, that would be consistent with the other Cultural Exchange assignments.

Thanks, I'll add this to the guide. Is it a liberated borg? What quality? Is it unique?

It's a medical assignment, and it can appear in various places. I've seen it in Alpha Trianguli and K7 system space.

Thanks, I'll add this to the guide.

Heh, I'm glad you figured it out. I see comments on this a lot, so I'll add this to the FAQ.

On Tribble I found a Saurian Cultural Exchange. I believe it was in Alpha Centauri. Don't remember any other details. Sorry

Yeah that's fine. I did make a slight typo though, it should read EDC instead of ECD here:

Oh and one more thing with the modified commendation tier rewards section:

The blues are 6000 dilithium, not 5000.