I did this with an Urban Decay eyeshadow cream palette. Hated the shadows, but the palette was really neat so I cleaned them out, sanitised it and re-used it. Now I have an 8 colour lipstick palette!
I like mixing colours that don't work well on me this way to get something better. A bit of lip balm or lipgloss or lip conditioner makes the texture a bit softer and helps it melt nicely at a lower temperature (I found there are problems if things get too hot).
I found you have to watch the temperature you heat things up to:
Too low and they did not mix well and it did not pour easily and looked lumpy afterwards.
Too high and you must lose some of the lighter oils because the lipsticks got drier, and you can even change the colour as well if it gets too hot.
I still do it with ends of lipsticks (so I can b2m them) or cheaper ones that I'm not sure I want to bother keeping.
Loads of fun if you've got some time to play around