Can my wife join me under my B-Visa as a trailing spouse in Thailand?

you first need to go to the immigration office where YOU get your extension, show your marriage documents see what certification they will need from the embassy of where ever you got married to be used here in thailand for an extension based on 'trailing spouse'

Once you have that figured out and get the documents certified, your wife would cancel the extension she's on at the office that issued her extension with termination paperwork from the employer, etc

After her extension is cancelled you'd go immediately to the immigration office where your extension was issued and apply for an extension of stay based on "trailing spouse" for 1900baht.

They should give her a 30 day under consideration stamp then after that an extension that's valid for the stamp you have (a matching extension). Then each year when you apply for your extension based on employment she'd apply for her extension based on trailing spouse..

It's going to come down to your immigration office and the documents they require to issue her a trailing spouse extension based on marriage to a foreigner who legally works here.

I do not think there's any issue cancelling her extension based on employment thru a BOI company and then applying for the year 'piggy back' (trailing spouse) extension if you have the correct paperwork