Hubs are standard width, which varies on intended use. For instance a road bike rear hub is a few mm narrower than a mountain bike hub.

If you are able to replace the freewheel to a 5/6/7 speed you will also have to change out your shifter and derailleur. There is a slight chance you can use your existing hub with a newer freewheel with 5 to 7 cogs.

By chance is your bike a Russian dating from the mid 1970's? I remember putting several of those together for the bike shop I worked at then.