If you mean "Playing a guitar through a keyboard amp" then yes you can, but it wouldn't sound great as you don't have a lot of the tone control functions from a guitar amp. (They are usually built into keyboards)
1. E/Acoustic guitars sound great through k/b amps. 2. Electric guitars do not, unless you're using the amp as a PA for a Line6 POD or other amp simulator. 3. Can you? Of course. Will it meet your needs? If acoustic, yes; if electric, probably not. More information as to your needs and equipment is necessary before I can provide a direct response.