Hello,
1. After I have configured UTEST sector with EB Fls, can I directly use Fls_Write()/Fls_Read() to read/write OTP data?
This depends on the driver capability. Simply check if the driver set up all the precondition to write to UTEST. Which would be the preparation to write to flash as usually ( I expect this is done) and then check if UTEST programming is enabled by

2. I saw in another QA that only 72 bytes in the UTEST area available for customization, If I want to write data to the OTP area that is larger than 72 bytes, is there a solution?
Yes, there are 16+56 bytes allocated for customer use.
You can possibly add the data after DCF stop record, but have in mind those data are not scanned by DCM so you need to read them by core, etc...
Also once you do that, have in mind in case you need add DCFs you wont be able unless you leave some space in UTEST for them. Before doing so make sure you fully understand how DCFs reading works.
I can't find the document S32K3xx_DCF_client.xlsx, could you please share it with me?
No I cant share it with you. It is embedded attachment in the reference manual:

Best regards,
Peter