It is possible to go from diocesan priesthood to religious life but not easy (if anything it’s easier to do the opposite). The priest would need to get a leave of absence from his bishop and be accepted by the superior of the order he wished to join. He would then have to follow the usual process for entrants into the order (novitiate, temporary vows, etc) before ultimately (and again with his bishop’s agreement) being incardinated into the order. For all religious, and Franciscans in particular, priesthood is secondary to their religious life - they are members of their order before they are priests.