Welcome to this user forum. @Abimeynell
You can add additional ones and pair them with the master one, but you cannot have two separate connection back to the home hub. It can be difficult to obtain single additional hotspots without having to buy another unit which you cannot use.
Which hotspot units are you using?
It can be difficult to get hold of additional
hotspots, without ending up with a spare master
In addition to what Keith has posted, is your garage on the same ring main (consumer unit) as your other powerline hotspots. If it is there should be no issues adding another one but if the garage is on a separate ring main (consumer unit) it is unlikely that it would work.