Yes, it's a very common practice. You can extend both the amount and the term, although once you have agreed this with the Bank a new document will have to be drawn up before a notary.
The most common reasons for requesting a mortgage extension are:
In the case of the amount
The need to finance new projects, such as renovation of the property, or to finance other assets that have nothing to do with it.
In the case of the term
The reason is usually to make the repayments more comfortable by reducing the instalment amount.