Add PAN No, TIN No and Tax Fields on Invoice Entity

I am using Microsoft Dynamics CRM 2013 Online.

Requirement is to have fields  -   Service Tax, PAN Number, TIN  Number, CST(Central Service Tax) and GST(Gujarat Service Tax) on the Invoice Entity.

These fields vary with the Location(Site) from where Invoice is getting generated.

So I thought of the Solution like, 

1.    I will add Site look up field on the Invoice Entity and put all these fields on the Site Entity Form. But CRM doesn’t allow to customize the Site Entity form.

Is that a good option to add them on the Territory though it is logical partition ? 

Please suggest.