Can an external recommendation (from a lab) be sent through Common App

<p>Can an external recommendation (from a lab) be sent through Common App or do I need to ask my P.I. to mail it directly to each of my schools. Would it be okay if I invited my P.l. to do a recommendation just as a teacher even though they weren't or would like recommendation limits prevent me from sending it in as supplemental. HELP?!</p>