If you are in a context in which you only mentioned big round numbers like 500000 or 3000000 and you suddenly need to read out 1000001, I'm afraid you cannot avoid some very cumbersome description like:

• Un million et un, juste un, pas cent mille
• Un million un, je veux dire un zéro zéro zéro … et un

On other hand, if you are reading a series, with detailed numbers like 154005, 3650084, etc. then “un million un” should be understood properly I think.

“Un million et un” is a bit odd but not unbearably so. If you make a short pause between “un million” and “et un” (and perhaps a brief hand gesture to explain “at the end”!), it could be effective.

If you are in a context in which you only mentioned big round numbers like 500000 or 3000000 and you suddenly need to read out 1000001, I'm afraid you cannot avoid some very cumbersome description like:

• Un million et un, juste un, pas cent mille
• Un million un, je veux dire un zéro zéro zéro … et un

On other hand, if you are reading a series, with detailed numbers like 154005, 3650084, etc. then “un million un” should be understood properly I think.

“Un million-et-un” is a bit odd but not unbearably so. If you make a short pause between “un million” and “et un” (and perhaps a brief hand gesture to explain “at the end”!), it could be effective.