This command is part of the TclX package.

lvarcat var string ?string ...?

Treats each of the strings as a list and concatenates them onto the list in var (which is created if it didn't already exist).

Compare with concat.

% package require Tclx
% lvarcat i "{" "abc" "123"
{ abc 123
% llength $i
unmatched open brace in list

It looks, to me, as if it treats each of the strings as a string, concatenating them together, and that if you want the result to be a list, you need to construct the arguments appropriately. Note also that each string is separated by a space.