For the general question of "can I submit a paper to arXiv that has already been published in a journal":
arXiv does not appear to have any policy against uploading a previously published article per se.
However, in their submission terms, arXiv requires that the "distribution of the [submitted work] by arXiv will not violate any rights of any person or entity, including, without limitation, any rights of copyright," and also requires that the submitted work "is not subject to any agreement, license, or other claim" that could interfere with arXiv's ability to host/distribute the work you're uploading. (If there are any other authors on this paper, they would also need to agree to post it to arXiv, as well, but that's secondary to the copyright/licensing issues.)
So whether you can submit a previously published article to arXiv depends primarily upon the terms you agreed upon/license granted to the publishing journal. The journal may have the exclusive copyright on your work - while many journals carve out explicit exceptions that allow authors to distribute copies of their work in certain circumstances, it is very possible that putting the whole text of an article published in a journal on arXiv is a violation of that copyright, and thus also against arXiv's terms of use.
It is possible that prior versions of a work which are NOT under copyright from the journal can be submitted to arXiv, but this is a somewhat murky area and you/arXiv might still have issues.
TL;DR: you need to know the exact terms of your agreement with the journal to know if you can submit the article to arXiv or not.