ventilation question.

Sorry this may need to be in grow room design set up but since Its pertaining to HPS I thought it may do. Basically I have a small hole in my wall formerly used for Ethernet and its about 3 or 4 inches in diameter, I haven't acquired ducting yet however my setup is gonna require about 6or 7 inch diameter ducting and was curious if there was a common piece I could grab to taper the exhaust end into the hole and if the airflow would be affected? As I was writing this I just realized I don't exactly know what end of the ducting the fan I need to get will be on if part of the ducting attaches to my overhead?

My question lacking? Logic? Lol

Fan out put, must match exhaust output with no down size tube...for length of exhaust...
Or else the fan will over heat, possible fire hazard, because of extra resistance that will build up as motor tries to over come obstruction...
MAYBE JUST EXPAND HOLE FOR DUCT TO RUN THROUGH...

Seems plausible there's just a metal frame around the hole that use to hold the Ethernet port that I could work around. Thank you for informing me of friction I was worried of something like that. So should I get a fan that has ducting attachable to both sides or is that the standard anyway?