Yes, and yes.
Be careful with this, you could be killed if you fall from that height.I have gone up many masts. I always try to rig two lines that will support me, one to the bosun's chair, a halyard; a second halyard to a harness that I wear. You'll need enough people and blocks and winches to do this, of course.If you can only use a single halyard, I still recommend a harness in addition to the chair, and tie the harness directly to the halyard line above the shackle with a rolling hitch, and tape it. This way, if the shackle fails you are still held up.I rig the bottom of the halyard through a snatch block to the anchor windlass - mine has a vertical capstan as well as the chain gypsie. You could also use your halyard winch, but I recommend the primary (jib sheet) winches. Make sure your mate on deck knows what to do, and you have an effective way of communicating.