RESOLUTION
Use template customization method, described in this article:http://download1.swsoft.com/HSPcomplete/3.3/release/doc/HSPC_SDK/template_based_customization.htm
Login page is defined by template login.tmpl, located in
/var/opt/hspc-root/skins/<skin>/template/HSPC/SM/CoreUI,
for example
/var/opt/hspc-root/skins/winxp_blue/template/HSPC/SM/CoreUI/login.tmpl
To customize login template, create directory /var/opt/hspc-root/custom/template/HSPC/SM/CoreUI
Copy /var/opt/hspc-root/skins/winxp_blue/template/HSPC/SM/CoreUI/login.tmpl there.
Modify file /var/opt/hspc-root/custom/template/HSPC/SM/CoreUI/login.tmpl
This customized login page will be applied to all skins.