You should be able to select items on the sheet and move them around. Is it that you cannot move them far enough to whichever side you are trying to move them?
Numbers on the iPad doesn't appear to have any settings for margins; at least I've never found them. Pages has them (Document Setup, after clicking the wrench) but not Numbers. I don't know what Numbers on the iPad does regarding margins with an imported Excel document.
There is a separate forum here for the iOS iWorks apps. You might get a better answer there.
EDIT: I created a test xlsx file with huge left and top margins and imported it. The table imported into iOS Numbers with the standard-sized Numbers margins. Maybe I didn't understand your question?
Margin settings are made in the Sheet Inspector. Open the Inspector, then choose the second icon (sheet) in the row across the top. You should see an Inspector pane similar to this one:
Since the procedure is likely different in the iOS version, I'll ask for this thread to be transferred to the iWork for iOS community.