Expelled from undergrad university for poor academic performance, can I go back there (years later) for Masters?
Keep trying!  I was on academic probation (at a Canadian university :-) ) after my first year of undergrad (2.88 gpa). Thankfully I did not get kicked out. I persevered and now have an MSc and PhD and have been doing medical research for many years. Don't take this as gloating, just trying to say anything is possible :-).

Now, it will depend a lot on the university or department, but if you can convince a faculty member in the department that you want to do research and that you have completely changed your attitude, maybe they will have some sway (at least with the department).  In my field, it is primarily a matter of convincing a faculty member (assuming the grades in the "lesser" university were good).  Again, this will depend on the department.

Alternatively, maybe you can make an appointment with the admissions department and make a case to them why you should be allowed back in to the university.  I suspect many would take your 7 years of experience as a growth experience.

Good luck!