Hi there, Eug,
If you have a copy of 'Simple Workshop Devices' by 'Tubal Cain' (the real one - aka Tom Walshaw) or can get access to a copy, have a look at page 106 onwards. This book is well worth having.
He described how to make holders for 'throw-away' endmills by modifying a Morse Taper drill-chuck arbor. He first mounts the arbor in the female taper of his lathe mandrel and machines a short parallel section in the Jacobs taper part. Then he reverses the arbor and holds it by that parallel section with the narrow end supported with the tail-stock centre. In this configuration, he turns a short parallel section in the narrow end of the taper, just long enough for the jaws of his fixed steady. He next withdraws the tail-stock, mounts the fixed steady and turns off the tang, faces the end and drills and taps for a draw-bar.
You could, perhaps, adapt this procedure to your tanged end-mills. That will depend upon the relevant parts being soft enough to be machinable. You might be able to omit the stage of turning the parallel section at the thick end of the taper if you can grip the business end of the mill, with soft packing pieces, in a collet or in a four-jaw chuck. There should be a centre drilling in the tang of the end-mill. And you need a fixed steady.
Tubal Cain's other book, 'Model Engineers Handbook', is worth having, too.
I thought Tubal Cain also listed the draw-bar thread sizes for various Morse tapers but that, I found, is in G.H.Thomas' book, 'The Model Engineers Workshop Manual', on page 159. This book, too, is worth having. (Please don't lecture me about omitted apostrophes - both the authors omit them!) He specifies ¼" BSW for MT1.