How to modify /proc/net/ip_tables_targets

[hypers]

I want to insert SNAT to the /proc/net/ip_tables_targets.

But the command:
> vzctl exec xxx sed -i -e 's/$/\nSNAT/g' /proc/net/ip_tables_targets
caused error "couldn't open temporary file".
And I cannot find any directories on /var/lib/vz/private/xxx/proc.

Any advices will help me.