Entities in Docbook 5 rnc

Hello,

I'm trying to upgrade our Docbook 4.2
DTD to Docbook 5 RNC.

In our version 4.2 DTD we included an
external entity file that listed

the conditions that we used for condition
attribute. Is there a way to link/include external files with the docbook
5 RNC?

For example, is it possible to have
a file (eg., myconditions.ent) that defines an attribute such as (e.g,.
db.iaconditions12.attributte) that

can be used in the definition of db.condition.attribute?

e.g., File myconditions.ent

db.myondition12.attribute =

    

    ## provides a standard
place for application-specific effectivity

    attribute mycondition12
{"asajtools"

         
              |"dbadmin"

         
              |"mobilink"

         
              |"programming"

         
              |"sqlref"

         
              |"sqlug"

         
              |"uldotnet"

         
              |"ulfoundations"

         
              |"rsguide"}

E.g., I'd like to do something like
the following in the Docbook 5 customization file:

db.condition.attribute = 

    ## provides a standard
place for application-specific effectivity

    attribute condition {mycondition12}

If this is possible, how should I reference
the myconditions.ent file  in the Docbook 5 RNC? 

And do I need to include a namespace
reference in the myconditions.ent file?

Thank you,

Kate