i've ripped an album with dbpowerAMP reference 12.3 to AAC. The file extension is .aac

*encoder: AAC (HE) SBR PS (Header: ADTS)
*ID tag: APEv2

foobar2000 plays the file correctly, but i can't tag the files.

what can i do?

If I remember correctly, ADTS and ADIF files do not have any tagging specification. Other software might let you put ID3v2 or APEv2 tags on such files, but it technically isn't valid. foobar2000 will not produce such files.

You need to use another container supported by foobar2000 that does support tagging, like MPEG-4 or Matroska. QuickTime (which is a superset of MPEG-4) also supports AAC audio streams and tagging but isn't supported by foobar2000. Stick with the MPEG-4 container, it's really the standard now, unless your files are destined for embedded devices or something like a cell phone.

thanks for your answers...otherwise if i put an aac file into a mp4 container i can't tag with fb2k, the file inside in.
then i supose i need to convert into another format?

I don't see how you would be putting the AAC file into an MP4 without converting it -- are you just renaming it or something?

Use something like mp4creator60 (or whatever is hot and new and does the task) and conversion should be easy.