There are some instances of allied crews fitting sandbags and extra track sections to their tanks to help protect from panzerfausts but that wasn't very effective. Most modifications weren't by the crew themselves but by special research divisions that tested a wide range of equipment. This led to such contraptions as the duplex drive for the Sherman tank which gave it amphibious capabilities and mine flail for clearing landmines n such. They were all developed by engineers.

Follow up question: did the soldiers often modify weapons in any way? What were they allowed to do with uniforms?

This video has a panel of tank historians discussing the fitting of extra armor to tanks. All sorts of improvisations were used - sandbags, track links, wood, even concrete. These were sometimes a deliberate effort by Ordnance officers in the field, and sanctioned from above, other times they were on the initiative of the crews. Sometimes this was permitted, other times it was not; Patton, for instance, was not a fan of improvised armor.

Crews were generally quite free to modify their tanks unless higher-up thought it compromised effectiveness. Indeed, some have speculated that the undeserved reputation of the Sherman for brewing up was because of British crews in the desert, who would pack extra ammunition around the crew compartment, so paranoid were they about running out.