[CentOS] Howto change arch for FirebirdSS rpm

Hello

I have this problem:

I make my own php rpms, base on src.rpm from CentOS but with my patches
(more options included). 

I have built it on my machine originally but now I want make it in mock
environment. I solved almost everything but Firebird support for 64bit
arch. Original RPM from http://www.firebirdsql.org/ are made with amd64
arch but my php is x86_64. And I don't know how to solve it.

.src.rpm for FirebirdSS and/or FirebirdCS is on available. 

I have this section in .spec
%if %{with_interbase}
%package interbase
Group: Development/Languages
Prereq: php = %{version}-%{release}, perl
Summary: A Sybase/MSSQL database module for PHP.
Provides: php_database
# BuildPrereq: /usr/include/gds.h     /usr/include/ibase.h
# BuildPrereq: /usr/include/iberror.h /usr/include/ib_util.h
# BuildPrereq: FirebirdCS >= 1.5.0
# "AutoReqProv: yes" don't work because of fault in Firebird RPMs
# # AutoReqProv: no
# BuildPrereq: FirebirdSS >= 1.5.0
# # Requires: /usr/lib/libgds.so
%ifarch i386 i486 i586 i686
BuildPrereq: FirebirdSS >= 1.5.0
%endif
%ifarch x86_64
BuildPrereq: FirebirdSS.amd64 >= 2.0.1
%endif

But mock failed with this:
DEBUG util.py:256:  No Package Found for FirebirdSS.amd64 >= 2.0.1
│54

I don't how to include amd64 arch rpm to my x86_64 repository. 
(my repo: http://ftp-hk.tmapy.cz/tmapy-twist/centos/4/os/x86_64/RPMS/)


I think there are two possible solution, rebuilt Firebird rpm with
different arch or include amd64 to x86_64 repository. 

Do you how to make it?

My questions:

1. Can I reconstruct .src.rpm from binary rpm or change arc in binary
arch? How?
2. How to include amd64 arch rpm to my x86_64 repository?

Thanks in advance

Pavel