Yes, it is possible, however that's not the best use of the platform really.
OOB you have Entra ID as authentication option, so your internal users can use that.
From a licensing point of view:
if they already have a Power Apps / D365 license, that will be considered as part of the user license
if they don't have a Power Apps / D376 license, you need to purchase Power Pages authenticated packs
For internal users, I would recommend looking into Model-Driven Apps or Canvas Apps depending on your data-source.
Can you give more details of your business use case?
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