If your credential contains the stamp of the Cathedral & Pilgrim Office on the inside front cover, and on the last page of sellos from your past Camino, the CREDENTIAL IS CLOSED, and cannot be used to support obtaining another Compostela. They do this to preclude folks from using disjointed Camino segments cobbled together to look like one legitimate length Camino.
The SOLE exception would be to use for sellos on the route from Santiago TO Finisterre or Muxia. These destinations do not qualify a pilgrim for a Compostela.
The Tourist Information (TI) offices at Finisterre and Muxia issue the Fisteriana and Muxiana certificates respectively as a form of official welcome and greeting to pilgrims visiting these towns. However, these are not Compostelas and have no relationship to the documents issued by the Cathedral at the Santiago Pilgrim Office.
You CAN use excess space in any credential to start gathering sellos at Santiago, and leading to Finisterre and Muxia.
Alternatively, you can go to the TI for Galicia, colocated at the Pilgrim Office to obtain a free guide to Finisterre & Muxia. This map-like document contains all manner of useful information for pilgrims walking from Santiago to the coast. It also had abundant space for sellos. It functions as a credential for this short route. Again, the TI Galicia Guide to the Camino Finisterre and Muxia is FREE.
I hope this helps.