[Supera]

Usa strcat: https://sampwiki.blast.hk/wiki/Strcat
Exemplo:

pawn Код:

CMD:dialog(playerid, params[])
{
    new string[128];
    strcat(string, "Seu texto");
    ShowPlayerDialog(playerid, 1, DIALOG_STYLE_MSGBOX, "Titulo", string, "Fechar", "");
    return 1;
}