The Library.xml can be imported from the Application Designer list tab. Near the top of all application XMLs including the Library.xml there is an ID tag that indicates which application to overwrite. This allows any application XML to be imported from the list tab, or even while in an application record.
Any changes to the Library.xml are at a system level and most likely will affect other applications. Most of the items in the Library.xml are shared among applications. To only affect the asset application there are a couple of strategies I have utilized.
First is to make a copy of the dialog that you want to modify in the Library.xml. After you make the copy you will have to change all the IDs to be unique including the dialog ID. The trick with this approach is you have to change the reference that calls the out of the box dialog to call the custom dialog, which can be tricky.
The second approach avoids this by reusing the same name but in the specific application's xml. For this approach you copy the dialog from the Libray.xml and then add it to the application's XML, asset.xml in your case. The IDs should already be unique in the XML so no need to change them. When the dialog is called it will call the one from the application.xml and not the Library.xml. Since you utilize the same names you won't need to change the references that call the dialog. You will also be able to open the dialog from application designer if you are more comfortable working there instead of in XML.