First of all, on the topic of CR (challenge rating), which you mention:
A CR equal to the APL (average party level) should, on average, cost the party about one quarter of their resources (resources in this case means hp and special abilities that refresh daily, like spells or a barbarian's rage).
Based on that assumption, a CR equal to APL+1 costs about one third of daily resources, a CR equal to APL+2 costs about half of daily resources, a CR equal to APL+3 costs about three quarters of daily resources, and a CR equal to APL+4 gets the PCs close to death and totally drained of daily resources.
Also, keep in mind that while three CR 1/3 monsters are a CR 1 encounter, 9 of them actually are a CR 4 encounter (see table 12-3 on p. 398 in the Pathfinder Core Rulebook).
As for your main question, since there are a lot of differences between Pathfinder stats and Starfinder stats, just taking PF monster stats and using them for SF can be a bit problematic and will require at least some tweaking.
But while there are no monsters in the CRB, lots of them can be found in Alien Archive. Many of those races can be used as PCs as well and the book also includes easy rules for creating your own unique aliens.
You can find a lot of monsters from Alien Archive on the starjammersrd.com website. Note that it's not as fully developed as the Pathfinder SRD site and the list of monsters will not have links to the actual monster pages, and the site's internal search is pretty broken. So when you find a monster on the page that looks interesting, go to Google and search up starjammersrd [monstername] and it should pull up a link for you.
I do recommend getting the book, but this should tide you over if you're running soon~
I used a few converted PF monsters in my campaign (based on the converting rules in the SF CRB) and have to admit they felt a little bit weal (compared to the Alien Archive ones).
So I would suggest to just use the monster as template and build them from scratch using the alien archive rules (they are really quick and there are already some "builder websites") or reduce the CR by one for converted PF monsters.
Can not seem to find any monsters in the SF crb. And i kinda need to add monsters to the dungeon i made for my group to die in.
Core Rule Book, Page 501, Legacy Chapter, Monster conversion.
According to the converting rules, you should add ~25% to the hp from PF to get the SF numbers, I'm not really sure if this is enough. (see pithica42s example)
Yeah, part of the problem is that in PF CR and HD are not necessarily tied to one another, like, at all. In SF, CR determines hitpoints, attacks, et al, while in PF much of that is determined by HD, not CR.