Yes, the policy management section of NSM can be pushed to either SRX or SSG devices. But there are some features that are specific to the platforms that would need to be converted.
I have a number of policies that either migrated to Junos or have a mix of SSG and SRX with the same rule set applied. You just need to avoid nat and build out the rest of the policy to be compatible for either OS.
Unfortunately, I've not located any documentation on this.
If you have a lot of changes for these differences, I would use the save as to make a copy of the existing policy to modify so that roll back is easier and testing in advance possible.
Differences:
NAT - All your NAT rules need to be removed for the SSG policy and created as a nat tab for the SRX. You may still need the security rule if the traffic is not permited by some other rule. The dffierence is that in ScreenOS your nat and security policy are on the same rule. In Junos they are in different tabs and rules. Junos nat rules also have a limit of 8 address objects and no groups. So you may need more rules with these limits. VIP, MIP and DIP all need to be converted.
Multi-service objects - there are a few service objects in screenOS that have a single service object with multiple ports on the object. These will show up in Junos as ONLY the first port. You will need to convert these to service groups.
Service custom time outs - In NSM the custom time out for Junos and ScreenOS are in different sections of your custom services. You need to fill in both. Also note that one is in seconds and the other in minutes so they are different numbers.