Trouble linking a text file into a text frame in Impress

[rtasch]

Link box is gray'ed out.
I want to link an unformatted text file into text frames in multiple Impress files so that the text can be edited from the source file, but formatted locally. Also need to resize the local text frame to fit the slide layout without changing text size etc. - normal text frame operation. Can't get the "link" check box active when inserting the file; using INSERT / FILE. I've tried using .txt, .ascii, .rtf, & .htlm files. Works fine if file is inserted without linking, but loose source editing capability.