Although Atticus doesn't support nesting an unnumbered list within a numbered list, you can create this look by combining the soft return (Shift + Return) with Unicode bullet points.
Examples that work in Atticus:
Bullet, black small circle (U+2022) •
Triangular bullet (U+2023) ‣
Black rightwards bullet (U+204D) ⁍
White bullet (U+25E6) ◦