How to i switch my iphone to manual sync on a different computer without loosing any music

I want to add music to my iphone, but i do not have my main computer available. I have itunes 10.6.3 installed, and once i connect my iphone i click "manually manage music and videos." Once i hit apply, i get a notice saying i can only have it synced with one itunes, and i can cancel the changes or erase everything and sync with the currect itunes. I only have a few songs on this secondary computer which i want to add, so i obviously dont want to do this.

What can i do to apply these changes, i feel it makes no sense that i have to erase everyhthing when i switch to manual sync, which is made for being able to add music from multiple computers.