I was interested in knowing whether questions on SAP MM module or others modules can be asked on this site
In general, I'd say that as long as your question is programming related, feel free to ask your question. If it isn't appropriate, the community will be quick to let you know.
But be aware that for extremely localized questions in that they require very narrow expertise to get a good answer, Stack Overflow might not be the best place to post. In those cases, it's usually better to turn to the community dealing with that specific technology. If there is one, that is.
You can, I there are other
ERP systems, like
JD Edwards, that have questions on SO. However the community is extremely under represented on SO that you probably will not get any quality answers. The best bet for these answers are vendor specific forums and communities.