It is not -po-, but the -si- of which makes it be an "If" statement (negative). Additionally, if it is -ki- then it becomes an "If" statement (positive).
Nikiimba, nitatunzwa (If I sing, I'll be rewarded)
Nisipoimba, sitatunzwa (If I don't sing, I won't be rewarded)
Tukikubali, tutafanikiwa (If we accept, we will succeed)
Tusipokubali, hatutafanikiwa (If we don't accept, we won't succeed)
-sipo- is actually the full prefix, and means "if not". Both -si- and -po- are needed in combination to make it a negative conditional.
My understanding is that -po- is different from the conditional -ki-, but -po- is also used as the negative conditional. If or when in English communicate basically the same idea in this case.
Msipoimba hamtakula (If/When you don't sing, you will not eat.)
-po- in the positive past/present/future is usually translated to when.
Mlipoimba nilicheza (When you sang, I danced) Mnapoimba ninacheza (When you sing, I dance) Mtakapoimba nitacheza (When you sing, I will dance) - for the future, -ka- is always put between -ta- and -po-, I'm not sure why but that's what my Swahili teacher taught me and how I always heard it used.