AP Invoices when created with ISuppliers portals are not creating Tax line information

Hi,

We are using Isupplier portal so that suppliers can create invoice and view invoice from their end.
Now we are facing the following issues
1) From the Isupplier portal page , there is no provision to enter tax data.Is that true that taxes cannot be entered from isupplier portal.
Please suggest if there is any way where in we can enter taxes.
2) When AP invoices are getting created , there is no tax line information getting created right now. So what we suggested for time being is to enter all tax details in the DFF from the Isupplier portal.
We were planning to take this DFF details and then update the existing invoices with a new tax line