How to add Properties[] properties in @Schema annotation?

ATM @Schema annotation doesn't allow to specify properties and additionalProperties part of the schema, if not by referring to an existing class via implementation, as you mentioned. We have this enhancement (annotations + processing) in our roadmap, but for the moment you can either apply the implementation workaround by e.g. defining some POJOs like the following:

...
class ExternalUserDTO {
 String foo;
}

class RoomOptionsDTO {
 public String bar;
}

@Schema(requiredProperties = {"user", "options"}
class UserAndRoomDTO {
 public RoomOptionsDTO options;
 public ExternalUserDTO user; 
}

...

requestBody = @RequestBody(
				content = @Content(
					mediaType = MediaType.MULTIPART_FORM_DATA,
						schema = @Schema(
							implementation = UserAndRoomDTO.class)

or use one of the extension mechanism, e.g. filters (https://github.com/swagger-api/swagger-core/wiki/Swagger-2.X---Extensions#filters)