Modify config.tcl to accomodate the legacy database, and to ensure that the legacy database is not used for standard OpenACS queries:

ns_section ns/db/pools
ns_param   pool1              "Pool 1"
ns_param   pool2              "Pool 2"
ns_param   pool3              "Pool 3"
ns_param   legacy             "Legacy"

ns_section ns/db/pool/pool1
#Unchanged from default
ns_param   maxidle            1000000000
ns_param   maxopen            1000000000
ns_param   connections        5
ns_param   verbose            $debug
ns_param   extendedtableinfo  true
ns_param   logsqlerrors       $debug
if { $database == "oracle" } {
    ns_param   driver             ora8
    ns_param   datasource         {}
    ns_param   user               $db_name
    ns_param   password           $db_password
} else {
    ns_param   driver             postgres
    ns_param   datasource         ${db_host}:${db_port}:${db_name}
    ns_param   user               $db_user
    ns_param   password           ""
}

ns_section ns/db/pool/pool2
#Unchanged from default, removed for clarity

ns_section ns/db/pool/pool3
#Unchanged from default, removed for clarity

ns_section ns/db/pool/legacy
ns_param   maxidle            1000000000
ns_param   maxopen            1000000000
ns_param   connections        5
ns_param   verbose            $debug
ns_param   extendedtableinfo  true
ns_param   logsqlerrors       $debug
ns_param   driver             postgres
ns_param   datasource         ${db_host}:${db_port}:legacy_db
ns_param   user               legacy_user
ns_param   password           legacy_password


ns_section ns/server/${server}/db
ns_param   pools              *
ns_param   defaultpool        pool1

ns_section ns/server/${server}/acs/database
ns_param database_names [list main legacy]
ns_param pools_main [list pool1 pool2 pool3]
ns_param pools_legacy [list legacy]

To use the legacy database, use the -dbn flag for any of the db_ API calls. For example, suppose there is a table called "foo" in the legacy system, with a field "bar". List "bar" for all records with this tcl file:

db_foreach -dbn legacy get_bar_query {
  select bar from foo
  limit 10
} {
  ns_write "<br/>$bar"
}