It depends on your device.
- For RUT2, RUT8, RUT9 source NAT can be configured in the Network → Firewall → Traffic Rules page.
- For TRB1, TRB2, RUTX source NAT can be configured in the Network → Firewall → NAT Rules page.
However, if you need the rule to apply to 10 IPs, you'll have to add 10 separate rules. To do it in one rule, you can use iptables. iptables rules can be added from the Network → Firewall → Custom Rules page. For example:
iptables -t nat -A POSTROUTING -s 192.168.1.5,192.168.1.15,192.168.1.25 -j SNAT --to-source 192.168.1.1
Replace the highlighted IPs with the IPs that you need. When adding more IPs simply separate them with commas (,).