Yes it is, but in doing so, your field tag now contains integer and character data, so you will have troubles shredding it later.

Select

ca.doc

From Main

Left Join Docs on Main.Id = Docs.Id

Left Join Tags on Main.Id = Tags.Id

CROSS APPLY(

SELECT [@name],

int_data AS [text()],

char_data AS [text()]

FROM (

VALUES

('Main', Main.Id, NULL),

('Tags', NULL, Tags.Tag),

('Docs', NULL, Docs.Doc)

) v([@name], int_data, char_data)

FOR XML PATH('field'), TYPE

) ca(doc)

FOR XML PATH('add'),TYPE

Drew