In case, user has a custom field it is not possible to map IDP custom field with custom field of SP. For example, I created a custom field named "Company" in IDP and SP. Now If I map Company field of IDP with Company field of SP, then after user import from IDP, custom filed of SP "Company" remains empty.
Steps to Reproduce:
1) Setup Two Liferay DXP SP7, one as SP and one as IDP using this document.
2) Under Attributes section of SP, map Company=Company
3) Create a custom filed named "company" in IDP as well as SP
4) Create a user in IDP and put some value in "company" field.
5) In SP, login with the same user, once logged in then goto the control panel (using Test user) and check the custom field "company" of imported user, it remains empty.
Liferay as SP should be able to map its custom fields with custom field of IDP.