Several possibilities depending on the services that may be available.
1) Issue the command
/msg chanserv vop [color:#66FFFF]#channel[/color] add [color:#66FFFF]nick[/color]
This does not require you to be in the channel when the nick enters in order for it to be voiced, but the network does have to support chanserv service.
2) Issue the command
/avoice [color:#66FFFF]nick[/color] [color:#66FFFF]#channel[/color]
This does require you to be in the channel, but no further scripting is required
.mode $chan +v $nick
The first two will only voice those nicks that have been previously specified. The last one will voice all nicks that enter the channel presuming that the client running the code has full ops in the channel.