Is it possible to bootstrap data into the default user table in OutSystems. As we have extracted data from a solution which we want to use in another eSpace.
Sure you can. You can create records in the ossys_user table. There are APIs available in the Platform.
I'm not sure accessing the table directly is a good thing to do.  I would perform the following steps.

1) Open any eSpace that references the User entity in the System eSpace (or create a temporary one that does)
2) Copy the User entity and paste it as a new entity - it will be given the name User2
3) Right-click on User2, click Advanced, click Create Bootstrap from Excel
4) After bootstrap code is created, delete the User2 - this will generate errors
5) Fix all errors by replacing User2 with User
6) Delete the code in the bootstrap that tests for existing records - if you skip this nothing will be added
7) Make other modifications if necessary. For example, I believe on recent versions of the platform you now need to encode the password.  You may also want to check for matching existing users before creating or updating.

The Employee Services component in the Forge bootstraps user data as well so you can use that as another model though looking at the code it is much more than you need.

You must be very careful with this!!  If you accidentally do something wrong to the User data you may have difficulties using it after the bootstrap.  Test thoroughly!!!

Hope this helps,