I'm 27 weeks pregnant with my first & our bassinet mattress is a wafer-thin, stuffing type material. The sleeping area feels quite hard, more than just firm. It is safe to get some custom made firm foam to add to create some sort of comfort? It feels like a wooden board at the moment.

Are you using a new bassinet, or is it pre-loved?
Red Nose recommends that a mattress (for a bassinet or cot) is firm, clean, well-fitting & flat.
While there are currently no Mandatory Standards for bassinets & mattresses, new standards have been developed & will become mandatory from Jan 2026. These Standards have been developed by Product Safety Australia (ACCC) & guidelines/recommendations are available to be viewed on their website.
https://www.productsafety.gov.au/consumers/keep-baby-safe/settle-baby-to-sleep-safely/bassinets-and-cradles-guide

You will see in this that a mattress for a bassinet should be no thicker than 7.5cm & the sides of the bassinet at least 30cm above the mattress.(New Mandatory Standards for height of sides will be 25cm)

It is not recommended to add an additional mattress to a bassinet (or a cot or portacot)
It is important that the mattress is firm well-fitting & flat (not elevated).
See also Red Nose information for bassinets. - https://rednose.org.au/article/bassinets