It is impossible to put mixin in such a sentence. What you can do is enable blocks inside mixin and use pikes | for plain text.
mixin myMixin() strong block p This is a sentence +myMixin() | with bold text | and this is the rest of it
What will be displayed:
<p>This is a sentence <strong>with bold text</strong> and this is the rest of it</p>
Craig source share