When you're already within a campaign process
It's easier and more cost efficient to include accessibility from the beginning of your campaign process, but if you want to add it in while amid the process or after the assets are finished, it's not too late.
Captions (Subtitles)
Closed captions can be added later - it's an additional file that doesn't require you to change your visual. You can supply this additional file to your media partners to enable it.
Audio Description
Audio description can be added later when the script has space for an additional voiceover. A partner can review your ad to determine what's feasible and produce this voiceover. It's an additional file to supply to your media partners, so you don't need to change your existing assets.
Alt Text
Alt text can be added after a post is published. On most digital platforms, you can update the copy that accompanies your published post to include this alt text labeled as '[Image Description]'. For those platforms that support alt text which is only available to screen readers, it's an additional field that can be updated later.
You won't need to change your assets or anything else already existing - this is a simple addition. You also don't need a special file format as the text will be copied and pasted into the form field on the digital platform's website.
Sign Language
Signed assets can be developed later and those will be separate files. Remember that sign languages are languages in their own right with their own grammar and syntax. They also very geographically and the local language should always be used. Your local provider of sign language services can assess how signing can be provided alongside your existing visual.
In markets like the UK, any new TV visual will need to go through the regulatory clearance process, which requires additional time and costs.