As long as the function has been previously defined and is in the same scope, you can easily do this. I'd recommend simply putting all the functions you want to use in your PowerShell profile. This way they will always be available. A variation, which is probably even a better approach, is to put all your functions in a single script, then dot source that script from within your profile script. This keeps your profile simple yet you still get all the functions loaded.
Jeffery Hicks
Microsoft PowerShell MVP
SAPIEN Technologies - Scripting, Simplified. www.SAPIEN.com
blog: http://blog.SAPIEN.com