If you have a bass player, or pianist, or guitarist playing the root (or even a singer singing), Bb, then you can play either Dm7 or F6.
Here's why this works (this is a bit of a review of some previous comments). Bbmaj9 is spelled Bb, D, F, A C. Without the Bb, you have D F A C, which is Dm7. Rearrange the letters of Dm7 and you get F A C D, which is F6. If you play either Dm7 or F6 and a bass player hits Bb, you'll both sound the chord Bbmaj9.
You may already know a bit about this and not know that you know it. When you play the open strings of the uke, you are playing C6 (C E G A) or Am7 (A C E G), depending on the context. If a bass player hits an F, then you both sound like Fmaj9.
Since the 6 chord is one of the easiest chords on the uke (the other being the m7), you can use it as a sub for any maj9. Whenever you see a maj9, play a 6 chord a fifth up from the root of the maj9 -- in other words, do this ....
When you see Fmaj9, play C6 (0000) and have the bassist hit an F.
When you see Gmaj9, play D6 (2222) and have the bassist hit a G.
When you see Amaj9, play E6 (4444) and have the bassist hit an A.
When you see Bbmaj9, play F6 (5555) and have the bassist hit a Bb.
And so on!
tl;dr If another instrument (bass, piano, guitar, voice) is hitting the Bb, you can play F6 (2213 or 5555) and it will sound like Bbmaj9.