Hi Nick~
I believe you can only export user-defined attack signatures.
Here's the chapter on Importing and Exporting Security Policies (not sure of your version):
https://support.f5.com/kb/en-us/products/big-ip_asm/manuals/product/asm-implementations-11-4-0/5.html
and
The chapter on Working with Attack Signatures:
https://support.f5.com/kb/en-us/products/big-ip_asm/manuals/product/asm-config-11-2-0/asm_attack_sigs.html
In the 2nd link, at the bottom of the page it gives the steps to export all user-defined attack signatures.
I know not exactly what you need but hope it helps.
ps