There isn't an Imperial Army. There are individual worlds Army units, some of whom are seconded to Imperial service.
According to
GT:Ground Forces there isn't an Imperial Army. There are plenty of references to the Imperial Army in CT, but all but a few of them can be handwaved away if you wave hard enough. However, there are one or two that clearly shows that the Imperial Army is every bit as much of an Imperial organization as the Scouts and the Navy.
"Over the centuries, the IISS became a major service of the Imperium, equal in stature to the navy [sic] and army [sic]." [Best of JTAS 5-8, p. 13]
The corollary is that the Imperial Army is a major service of the Imperium, equal in stature to the Navy and the Scouts.
Personally, I believe that if there wasn't an Imperial Army, someone would have to invent it. That is to say, the Emperor would have to ask one of the existing armies under his control to assume various duties that would not be delegated to individual worlds or even duchies. If there wasn't an Imperial Army, the Army of the Domain of Sylea or the Army of Core [Sector] or the Army of Capital would function as one. If you have a job to be done and no one to do it, you get someone who IS there to do it instead.
A historical example of this phenomenon are the British Horse Guards back in Napoleonic times that performed the same functions for the British army that the Admiralty did for the Royal Navy because Britain didn't have a 'Generalty' and the Horse Guards were stationed close to the Government, so they got to do the work instead.
Anyway, if you don't accept GT as canon, then there definitely is an Imperial Army. If, like me, you do accept GT as canon, you have a problem.
Hans