You don't "convert to IMAP format". mbox is a format. IMAP is a protocol. What these tools do is: read mbox, take each message, and upload it via IMAP to an IMAP server (in your case Gmail). You can specify into which IMAP folder messages go, etc. Might be a bit finicky to get it going, but anything is better than wobbly drag & drop. (Oops, I dropped too soon; disconnected during drop -- which messages were synced?; have I dragged that or not?; etc. You get the point. :-)
You don't "convert to IMAP format". mbox is a format. IMAP is a protocol. What these tools do is: read mbox, take each message, and upload it via IMAP to an IMAP server (in your case Gmail). You can specify into which IMAP folder messages go, etc. Might be a bit finicky to get it going, but anything is better than wobbly drag & drop. (Oops, I dropped too soon; disconnected during drop -- which messages were synced?; have I dragged that or not?; etc. You get the point. :-)