[T. O'Heir]

"...doubt a tailor would do it..." You're right. Tailors work with cloth, not leather.
The second picture doesn't show much of anything. Can you post a better picture of what you want?
"...No Tandy..." They do mail order. I don't think I'd bother for a one time modification though. Sewing leather has a learning curve. Do not cut anything. Leather isn't sewn like cloth.
Go to a tack repair shop(Not just a tack shop. They sell tack and can be a bit daft. It'd be best to talk to the guy who repairs tack at your local race track. Those guys work at fixing leather and usually aren't easily scared by a holster. Buddy of mine did it for years. Very little I asked him about phased him. Wouldn't give me his industrial sewing machine when he packed the business in though, but he did sew some thick canvas for me.) or shoe repair place(takes a lot to phase those guys too).
It's be less fuss to just buy the holster you want than it would be trying to modify a holster though.