Like most things you can just fill it with $chr(160). i.e.

tab "C            ", 4

Alternatively, to save you copy/pasting them in.

alias dtabs { did -a $$1 $$2 $+($did($$1,$$2),$str($chr(160),$$3)) }

Syntax; /dtabs $dname [color:red]object-id spaces-required[/color]

i.e.

dialog test {
title "mIRC"
size -1 -1 110 100
option dbu

tab "m", 1, 5 5 100 90
tab "I", 2
tab "R", 3
tab "C", 4

button "m is for ... wink", 11, 30 50 50 24, ok tab 1
button "I is for Internet", 12, 30 50 50 24, tab 2
button "R is for Relay", 13, 30 50 50 24, tab 3
button "C is for Chat", 14, 30 50 50 24, tab 4
}

On *:Dialog:test:init:*:{
dtabs $dname 2 10
dtabs $dname 3 10
echo done.
}

/dialog -m test test

Eamonn.