Hello,
Is it possible to convert an external .mtlx file into node format?

Yes, LOPs can do that for you. It's all ready built-in.

Enter the Stage context and switch to the Solaris viewport layout.
Drop down a LOPs File node and browse to the .mtlx file. (NOTE: You'll have to explicitly browse using the asterisk wildcard symbol for any MTLX files to appear in the dialog window.)
Drop down an Edit Material Network node and pipe in the file.
Drag the name of the MaterialX graph into the Material path.
Click Load.
Dive inside twice, and you'll discover your mtlx material network that you can copy to the /mat context for non-LOPs rendering.

really helpful!