How can I add extra properties to document metadata field types in Liferay 6.1?

I'm new to Liferay and I have the following situation:

I would like to add extra properties to document metadata field types in Liferay 6.1.20.

For example, I would like to add property 'readonly' to select boxes (see image), as it is a user requirement to show some controls disabled.

Should I use the Expando functionality, a jsp override hook or another approach?

I think Expando functionality, a jsp override hook are a good approach.But using Expando you cannot create it as mandatory field.