Hi Jordi,
Thank you for using Syncfusion products.
At present, we do not have support for converting an existing PDF document in to PDF/A1B conformance document, also this conversion should handle some cases like font embedding, converting the document color space and other cases associated with PDF/A1B conformance to pass the conformance test.
We have created the feature request for the same, we will let you know once this feature is implemented.
Please let us know if you need any further information in this.
Regards
Surya Kumar