THe word is preliminarily -- it's the adverb which corresponds to the adjective 'preliminary.
They have come up with some preliminary steps to solve the problem (adjective)
We have preliminarily come up with some steps to address the issue. (adv)
Preliminarily, it was thought that the drug would have no major side effects. However, after rigorous testing, it was found.....
Yes, you could use it -- it would be fine in academic or formal writing.