you can add your own icon at top of blackberry main screen using ApplicationIndicator[1]

ApplicationIndicator class
To add/modify/remove notification icon we can use ApplicationIndicator class:

Represents an application indicator that consists of an icon and optional numeric value.
The icon is placed in indicators area along with others such as new mail counter, calendar reminders, missed calls counter, security status and others.

Application can register only one indicator and has to register it every time the phone starts.
All indicators are cleared upon phone restart.
Indicators may be not visible if there will be no space on statusbar
Indicator dimensions depend on current theme and phone screen size.
The icon size varies from 10×10 up to 24×24.
If indicator is initialized with a big icon then an exception is thrown.
Indicator can have optional numeric value that usually serves as a counter.
The value can be negative, 0 or positive.
If indicator is used as a counter it’s better to hide it when its value is equal to 0.
If indicator value is too big, it will display ’99+’ or ’999+’
Add notification icon
For this we should create instance of ApplicationIcon:

Application icon class is used in conjunction with application indicator and application messages.
It incapsulates image and its painting properties.

EncodedImage mImage = EncodedImage.getEncodedImageResource(“indicator_icon.png”);
ApplicationIcon mIcon = new ApplicationIcon(mImage);
Then we should register ApplicationIndicator with ApplicationIndicatorRegistry:

ApplicationIndicatorRegistry.register(icon, iconOnly, visible)
icon – an icon to be displayed in the indicators area
iconOnly – specifies if indicator should have icon representation only and no value
visible – specifies whether indicator has to be visible initially

ApplicationIndicatorRegistry reg = ApplicationIndicatorRegistry.getInstance();
ApplicationIndicator indicator = reg.register(mIcon, false, true);
Modify indicator icon and value
To modify icon, use ApplicationIndicator.setIcon method:

ApplicationIndicatorRegistry reg = ApplicationIndicatorRegistry.getInstance();
ApplicationIndicator appIndicator = reg.getApplicationIndicator();
appIndicator.setIcon(icon);
To modify number value, use ApplicationIndicator.setValue method:

ApplicationIndicatorRegistry reg = ApplicationIndicatorRegistry.getInstance();
ApplicationIndicator appIndicator = reg.getApplicationIndicator();
appIndicator.setValue(value);
If very big value is provided then indicator will display ’99+’ or ’999+’.
To hide or show indicator, use ApplicationIndicator.setVisible

Unregister ApplicationIndicator
Most likely you don’t want your app indicator to stay in statusbar, it’s better to ApplicationIndicatorRegistry.unregister() it on app close.

[1] http://stackoverflow.com/questions/1465224/blackberry-how-to-use-notification-icon-in-statusbar