Both will play in Wellington's season opener against Canterbury on November 9 and Auckland a week later.
England all-rounder Chris Woakes will take over from Tait for the eight remaining matches in the HRV Cup and Bangladesh opener Tamim Iqbal will also play for Wellington later in the competition.
"The addition of two experienced T20 specialists in Shaun and Cameron will add significant value to the Wellington Firebirds for these two important matches," Cricket Wellington chief executive Peter Clinton said. "Express bowlers like Shaun only become available very rarely, while a seasoned T20 player like Cameron will enhance the balance and composure of the side."
"The Wellington Firebirds have nothing short of an All Stars look about it, with Tait and Borgas joining the likes of [Jesse] Ryder, [Mark] Gillespie, [Andy] McKay, [Grant] Elliott, [Luke] Woodcock and [Luke] Ronchi. The team is primed and ready, and we're looking for some great local support to really get the HRV Cup season underway in style."