I am looking for a tool or a way (.NET) to add custom XMP fields. Also, can someone explain the purpose of needing to know if the XMP tag is a textfield, textarea or a select?
XMP is written inside files as an XML packet or as a separate XML file. The XMP specification uses a subset of RDF/XML. So you could look at (RDF/)XML manipulation tools.
For embedded XPackets however, the packet length needs to be calculated and written at the start of the packet, so it may help to have a purpose built library. Adobe provides an XMP SDK (C++) for that.
XMP supports several content types for fields, like Text, Number or URL. Text fields, for example, could be restricted to values from a controlled vocabulary, for which it may make sense to use a select or dropdown form element in a GUI.