can i map a custom field to a standard field

There is a standard field called department in contacts module I have tried to map a custom field I created in the leads module as this field was not there. Unfortunately, it won't let me map the two fields doesn't show up in the drop down list, i had tried to ask this question to Daisy our account manager but she does not know.

I am not sure if there is an issue or that you cannot map custom fields to certain standard fields?

Thanks for your input.

Help