PHPstorm plugin create statement

PhpPsiElementFactory is located in php.jar. While it's an internal class and subject to change, you can still use it.

final String text = "$qb->addSelect('a');";
final Statement statement = PhpPsiElementFactory.createStatement(getProject(), text);
final PsiElement groupStatement = method.getLastChild();
if (groupStatement instanceof GroupStatement) {
  final PsiElement closingBrace = groupStatement.getLastChild();
  if (PhpPsiUtil.isOfType(closingBrace, PhpTokenTypes.chRBRACE)) {
    groupStatement.addBefore(statement, closingBrace);
  }
}

Alternatively, you can use PsiFileFactory, which is a part of the IntelliJ platform API, directly.

final String text = "$qb->addSelect('a');";
final PsiFileFactory psiFileFactory = PsiFileFactory.getInstance(getProject());
final PsiFile file = psiFileFactory.createFileFromText("temp.php", PhpFileType.INSTANCE, "<?php\n" + text);
final PsiElement groupStatement = file.getFirstChild();
if (groupStatement instanceof GroupStatement) {
  final PsiElement statement = groupStatement.getLastChild();
  if (statement instanceof Statement) {
    // TODO here's your code
  }
}

"this file is included in my php-openapi.jar"

I guess you mean its NOT included...

com.jetbrains.php.lang.psi.PhpPsiElementFactory#createStatement is indeed the most safe way to create a statement from text.

Unfortunately some of important classes are not yet moved to php-openapi.jar.

As to this one - you can use it quite safely, its not likely to disappear/move/break too often.