it is possible to put a different admincp password from login password?

Not that I'm aware of, without modding anyway, but there's a very, very simple workaround.

Use one 'dedicated' user ID for admin tasks only. Never post on the board with it, except maybe for official announcements.

Create another ID for posting and moderating.

In IPB 2.2 you can create an .htaccess file for the ACP to do this (and user server-level secondary password which wouldn't be stored in the database etc.)