Hello @Marta_Agraz,
Unfortunately, you are not able to attach the same piece of content (such as a blog) to more than one Topic in the “SEO” section of HubSpot (where all of your pillar pages and topic clusters are). Theoretically, you could still go into the blog post itself and edit to add a link to the pillar page(s) you want.
I believe the reason why HubSpot doesn’t allow you to link content to multiple Topics (pillar pages) is that this can confuse search engines when trying to determine which of your pages are related (if it sees the same page linking to multiple pillars, it could be uncertain which topic the page should fall under). I’m not 100% certain on that, but I think that could make sense, seeing as how search engines can be very strict with the way they analyze, interpret, and rank webpages and content.