Billiat joined The Brazilians from Ajax Cape Town in 2013 on a three-year deal, with the club activating a one-year option at the beginning of last year.
After winning the Absa Premiership Footballer of the Year for 2015/16, the Zimbabwean international also played an integral part in helping Downs to their maiden CAF Champions League title and more recently the CAF Super Cup crown.
Despite previously being linked with a move away from Chloorkop, with a number of clubs abroad reportedly interested in his services, Mosimane has revealed that they are looking to tie down their 26-year-old star.
"We're going to look after him [Billiat]. He's important for us. We have to keep him," Mosimane tells KickOff.com.
"All the important players need to be [taken care of]. We need to look after important players [in the team] – Khama is important for us, we have to look after him.
"[Contract] negotiations are already going on for Khama – he's like our [Philippe] Coutinho. Negotiations are going on and we'd love to keep him, if he really wants, why not? He'll stay."
Regarding a possible move abroad, Mosimane insists they will not stand in Billiat's way should "a proper offer" come in, and adds that no discounts will be allowed to European teams with bigger budgets.
"It's simple. We will release Khama if a proper offer comes in, we won't stand in his way," adds 'Jingles'. "When Stoke wanted the boy from Al Ahly – Ramadan [Sobhi] – they paid.
"You have to pay. Discounts? No discounts. Why do we give discounts to Europe? Europe must empower Africa. How can we give discounts to people who operate in Euros, who have got more budget than us? No.
"We want players to go to Europe. We've sent Bongani [Zungu] away; we've sent Keagan [Dolly] away, so I think the younger ones know now – come to Sundowns, you'll have an oppotunity to play in Europe."
He continued: "Do you want to tell me why Coutinho renewed his contrat at Liverpool? Because they look after him. He's not stupid, he knows he can go to Real Madrid.
"So Khama must stay in South Africa until somebody comes and knocks on his door, and we will release him. Why not?"
Billiat is amongst three players at risk of sitting out Sundowns' Absa Premiership clash against title rivals Bidvest Wits on Saturday, after suffering an injury in their last clash against Platinum Stars in midweek.