There is no such command. It may be possible through server side with a sourcemod plugin.
There is one workaround to achieve this behavior, if you use
Advanced Commands plugin. Set cvar
mp_freezetime 0
. And then execute command
sm_freeze
or
!freeze
.
Example:
!freeze @all <number of seconds>
Then all the players will be frozen until given seconds, and all the other actions will be allowed: jumping, shooting etc. Keep in mind that round is already started, if you use !freeze command. So increase the round timer based on freeze command value.
If you don't want to use command everytime, then you will need a plugin to do this automatically.