How can I mount a 25.4 mm seat-post on a frame that needs a 27.2 mm seat-post
There are seatpost shims available for a few dollars. You should get them easily in your local shop or online.

I use an aluminum shim in one of my bikes with exactly the dimensions you need, it works flawlessly and looks very clean. I got it for about 3.50 € (5$).