This is possible for Partner, Product, Invoice. You use the xml-info field. This field can contain any kind of data ; xml is the wished format since you can provide that way structured or semi-structured datas. For instance, in european countries, we can add there the intra-EE VAT number since it is not provided in the general Partner prototype. This info can be used on the client side : for instance, you can get it back to print it in an invoice when you customize invoice.xsl