Hi vcamposp,
Yes, we can add new fields to Contact entity and filter records based on them.
Adding Customization to entity in Dynamics 365 Marketing is same to what we did in Customer Engagement.
1. Open Advanced settings menu:

2. Settings > Customization > Customize the System

3. We might see a dialog which ask us whether to edit in modern UI.
Cancel: (keep in classical UI)
Entities > Contact > Fields > New > Save


Then our new field will be available as filter.

Ok: (optional, in modern UI)
Open Contact entity

Add field:

Regards,
Clofly