Can a widowed or divorced kohen who already has children marry a woman convert who cannot have children (eg, because of a hysterectomy or being postmenopausal)?
The pasuk says:
"אִשָּׁה זֹנָה וַחֲלָלָה לֹא יִקָּחוּ וְאִשָּׁה גְּרוּשָׁה מֵאִישָׁהּ לֹא יִקָּחוּ כִּי קָדֹשׁ הוּא לֵאלֹהָיו" "They shall not take a woman that is a harlot, or profaned; neither shall they take a woman put away from her husband; for he is holy unto his God"
The gemara discusses what is meant by "harlot" and concludes that it is a convert. It is important to understand that the word "harlot" does not have the same meaning as it does in our every-day language. It is not implying that the convert was anything bad.
Nowhere is there any mention of an exception.
The whole point is not the children but the purity of the Kohen himself.
No he cannot. It does not matter that she cannot have children and that he already has children and is divorced/widowered.