If you plan on going to school in the United States, you will need a student visa FIRST.
Then you will need to contact the University you wish to attend and speak with their International Students advisor.
No, you cannot challenge the RN Board with an LPN certificate. You will need to go through an ACCREDITED RN school first, THEN sit for the NCLEX-RN exam.
Hope this helps.
Also, you may find more answers to your question(s) if you post this in the INTERNATIONAL NURSES forum.