You can link up an account with a local entity.  Here is an example from the demo server

Add a user via apache password store ( or any other Authentication method ) . In this example we will create an account named "test_local_entity" in the authentication source.

[mchyzer@i2midev1 ~]$ sudo htpasswd /etc/httpd/conf.d/users.pass test_local_entity
New password: 
Re-type new password: 
Adding password for user test_local_entity

You also need to expose this account to grouper via a Subject API.
Fortunately Grouper has a built in Subject API to expose local entities created in Grouper. You can use that feature if you don't have another Subject API that would return the account name via a Subject Identifier lookup.
Here are a few screen shots of creating a local entity in a "special folder" (path to the folder is used in the configuration later) created to hold local entities for use as Grouper Web Service accounts.

Note the 'Local entity name': needs to match the name of the account.

Add it to the WS group. 

Note: It is best practice to never add a Subject directly to a policy group. ( The webServiceUsers group is a policy group. )

Configure a prefix on logins on WS: in to only select local entities from the folder where you store local entities for use with Grouper Web Services.
NOTE: Don't forget to KEEP the trailing ":" seperator. This string is used as a prefix string that is concatenated to any "username" that is returned from any authentication source.

# prepend to the userid this value (e.g. if using local entities, might be:    etc:servicePrincipals:   ) = etc:servicePrincipals:

Hit a link:  login as   test_local_entity   and    whatever_pass