add function to a wordpress plugin

i have a plugin that i need to add a function to, that replace a YouTube link on a post string fetched from rss feed with the video embedded on one line. and all the excerpt fetched from the feed need to be under the youtube and all right aligned with RTL direction. this is the function: function youtube($volcado) { if( ereg('\[youtube=',$volcado) ) { //YOUTUBE CODE $posicion=stripos($volcado,"[youtube="); $volcado1=substr($volcado,0,$posicion); $posicion=$posicion+9; $volcadotemp=substr($volcado,$posicion,strlen($volcado)-$posicion); $posicion2=strpos($volcadotemp,"]"); $enlace_video=substr($volcadotemp,0,$posicion2); $volcado2=substr($volcadotemp,$posicion2+1,strlen($volcadotemp)-$posicion2-1); $posvideo=stripos($enlace_video,"watch?v="); $codigo_video=substr($enlace_video,$posvideo+8,strlen($enlace_video)-1); $enlace_video="[login to view URL]".$codigo_video; $codigo_video=""; return $volcado1.$[login to view URL]($volcado2); //FIN DE YOUTUBE CODE } else { return $volcado; } }